1. Field of the Invention
The present invention relates to organic electrolytic solutions for improving flame resistance and charge/discharge properties and to lithium batteries including the same.
2. Description of the Related Art
As demand increases for lightweight, portable high performance electronic devices such as camcorders, mobile phones, and laptop computers, research is being conducted into batteries used as power sources for such devices. In particular, rechargeable lithium secondary batteries have 3 times the energy density per unit weight of Pb storage batteries, Ni—Cd batteries, Ni—H batteries, and Ni—Zn batteries. In addition, lithium secondary batteries can be quickly charged. Therefore, research into and development of rechargeable lithium secondary batteries are increasing.
In general, lithium batteries operate at high operating voltages so that conventional aqueous electrolytic solutions cannot be used due to the violent reaction between the aqueous solution and the lithium contained in the anode. Accordingly, lithium batteries use organic electrolytic solutions prepared by dissolving lithium salts in organic solvents. To that end, organic solvents having high ionic conductivity, high permittivity, and low viscosity are desired. However, it is difficult to obtain organic solvents having these properties. As a result, lithium batteries use mixed solvents containing one organic solvent having high permittivity and another organic solvent having low viscosity.
When a lithium secondary battery uses a carbonate-based polar non-aqueous solvent, the reaction the electrolytic solution with carbon acting as an anode requires excess charges during initial charging. As a result of such an irreversible reaction, a passivation layer (such as a solid electrolyte interface (SEI) membrane) is formed on the surface of the anode. The SEI membrane allows the battery to be stably charged and discharged without further decomposition of the electrolytic solution. The SEI membrane also acts as an ion tunnel through which only lithium ions pass, and prevents cointercalation of the organic solvent (which solvates lithium ions and moves with the lithium ions into the carbon anode), thereby preventing a breakdown of the anode structure.
However, during charging and discharging, the lithium battery is repeatedly subjected to high voltages of 4V or greater. Under such conditions, the SEI membrane (formed of only a polar solvent and a lithium salt) cannot retain the ideal properties described above. That is, the SEI membrane cracks, and thus an insoluble salt precipitates inside and outside of the anode, and gas is generated as the solvent is continuously reduced. Accordingly, the internal pressure of the lithium battery increases and the structure of the lithium battery cracks, making it possible for the electrolytic solution to leak. Furthermore, due to the leaking electrolytic solution, the lithium oxide at the cathode can be exposed to moisture in the air, thereby igniting the lithium battery. In addition, when the battery is frequently exposed to overcharge conditions, the battery becomes overloaded, causing an exothermic reaction to occur therein. When the temperature of the battery is greater than a predetermined temperature, high temperature ignition can occur. Such high temperature ignition is primarily caused by the electrolytic solution.
These problems have been addressed by adding a flame-resistant agent to the electrolytic solution. One such flame-resistant agent is an alkyl phosphoric acid ester ((RO)3P═O). Such phosphoric acid ester compounds have good flame resistance but can be oxidized or reduced depending on the type of anode used. As a result, an excess amount of phosphoric acid ester compound should be used. In particular, when a graphite-based electrode is used as the anode, use of an excessive amount of the compound leads to a substantial decrease in the charge/discharge efficiency of the battery.
Accordingly, there is a need for an organic electrolytic solution having flame resistant properties (self extinguishing properties) and excellent charge/discharge properties which can be used to manufacture a safe and reliable lithium battery.